Motor control systems are used in the oil industry for the protection of electric motors employed as prime movers in sucker-rod type pumping units Such electric motors typically take the form of three phase AC motors although other polyphase motors, e.g., two phase or four phase, may also be employed. Sucker-rod pumping units are widely used in the oil industry in order to recover fluids from wells extending into subterranean formations. Such units include a sucker-rod string which extends into the well to drive a downhole pump and means at the surface of the well for reciprocating the rod string. Typical of such units are the so-call "beam type" pumping units in which the sucker-rod string is suspended from a walking beam which is pivotally mounted on a Sampson post and driven by an electric motor.
The load on the electric motor varies widely during each pumping cycle and it is a conventional practice to monitor the operation of the unit and to shutdown the motor upon the occurrence of an unacceptable fault condition. For example, U.S. Pat. No. 3,778,694 to Hubby et al discloses a system for detecting a "pump-off" condition by monitoring the load on the motor during the downstroke of the pumping unit. When the system detects a motor load which is abnormally low in comparison with a predetermined standard, it acts to remove power from the motor. In the Hubby system, the power supply is three phase AC power and a timing unit is connected across one phase of the power supply. After a pump-off condition is determined and the motor shutdown, a switch in contactor relay circuit is periodically closed by the timing system in order to restart the pump.
Pump-off control can also be effected by the simple expedient of sensing the motor speed and generating a signal representative thereof. When the motor speed signal exceeds a set point value indicative of an approaching pump-off condition, the motor is automatically shutdown. A more complex system for controlling the operation of a well pumping unit based upon pump-off conditions and other fault indications is disclosed in U.S. Pat. No. 4,490,094 to Gibbs. In Gibbs, instantaneous speeds of revolution for the motor are determined and applied to arrive at a value of at least one selected parameter which is used to implement a control action. The selected parameter may be power output, modified average current, power input, thermal current and power factor for the prime mover along with transmission unit torque, instantaneous polish rod loading, or total polish-rod work.
Yet another protection system, which is especially useful in the control of well pumping unit prime movers to avoid stall conditions, is disclosed in U.S. Pat. No. 4,695,779 to Yates. In this system, a suite of stall condition set points are established corresponding to the torque mode of the motor. The torque mode of the motor is sensed to select the appropriate set point and a motor speed signal is compared with the set point to sense an approaching stall condition and shutdown the motor.
After a pumping unit prime mover is shutdown, it is a conventional practice to use a time delay restart system to again start the motor. A time delay may be provided for any one of several reasons One reason is to provide a sufficient time period to accommodate correction of a pump-off condition as described in the aforementioned patent to Hubby. Yet another reason is disclosed in U.S. Pat. No. 3,191,114 to Reed wherein time delay motor starting systems are employed to place a plurality of pumping units on line sequentially in order to avoid the cumulative effect of simultaneous starting currents. Time delays are also conventionally employed when a pumping unit electric motor is shutdown because a low voltage condition occurs or where a phase imbalance condition results from a disparity between the phase voltages provided to a the polyphase motor. For example, phase imbalance may be determined by monitoring the voltage supply to the motor to maintain a cumulative running average of the three voltages across the three phases. In the event of an imbalance between the highest and lowest voltage exceeding a predetermined set point, the motor is shut down. Appropriate timers are activated upon shutdown and the motor is again restarted after an appropriate time delay and continues running so long as the low voltage or imbalance condition does not then exist.
Whereas shutdown faults such as phase imbalance, low voltage or pump-off conditions are employed to implement time delay restart actions, other faults such as excessive motor temperature serve to lock the prime mover motor in the off or shutdown condition where it cannot be restarted without operator intervention such as by a manual reset. For example, it is common to provide the motor with a thermostatic control based upon one or more motor winding thermostats. When a thermostat opens due to excessive motor heat, a relay latches the motor control in the "off" position, requiring a manual reset before the motor can be restarted.